The present invention relates to the operation of processes involving the interaction of a gas phase with a liquid phase, e.g., gas-liquid contacting, such as in fractional distillation in columns containing contact trays, evaporation, aeration, and mixing by flow through orifices, and injection into bubble column reactors.
Distillation is a common process in the petrochemical industry to efficiently separate chemical compounds. It may be defined as the separation of the constituents of a liquid mixture by partial vaporization of the mixture, followed by separate recovery of the vapor and liquid residue. Distillation towers, such as pipestills, contain a vertical distribution of "trays". The fluid, from which the separation of the vapor is made, flows over the trays driven by gravity. The trays contain holes through which the released vapor can flow to be separated eventually into a vapor stream exiting at the top of the tower. In some trays the gas flows through the liquid in the tray through "bubble caps" or "float valves", rather than "holes".
In some petrochemical operations, involving the interaction between a gas and a liquid, trays are also used to separate the gas stream from the liquid stream. In other operations such as bubble column reactor, it is important to maximize the contact between a gas or vapor and a liquid. In all these examples, there is a delicate balance in flow and state variables such as temperatures, pressures and flow rates in order to achieve the desired operation. In some cases mechanical components such as spargers or trays can operate improperly. In all these cases a critical component of the process is a finely dispersed mixture of gas and liquid which we call a "bubbly medium".
In what follows, distillation shall serve as the preferred embodiment, but the current invention can relate to any operation where the unit contains a significant region where the desired operating state is that of a "bubbly medium" such as a bubble column.
In a pipestill, under ideal conditions, there is a clear separation of phases between what is flowing on the tray (a bubbly medium consisting of liquid plus rising vapor bubbles); what is flowing upward between trays (vapor or gas) and what is flowing downward through the "downcomers" (liquid). FIG. 1 is a schematic of a tower and shows the tray configuration. The trays are arranged in decks. FIG. 1 shows three decks with a number of trays within each deck.
Maintaining such a desired flow state is difficult since it involves setting pressures differences at each tray such that vapor or gas is flowing upward through the holes in the tray and not liquid downward. Similarly, at the "downcomers" it is important that the liquid flows "down", and that the gas or vapor does not flow "up". Another problem that could occur is the generation of "foam", i.e. a froth of bubbles that leads to flow disturbance. In addition, the liquid level on the trays could be higher or lower than optimal due to improper design, excessive feed rates, or poor vapor/liquid separation. FIG. 2 shows a schematic of the flow state in a distillation column. FIG. 2A shows a normal flow state. FIG. 2B shows an unstable flow state.
In addition to the problem of setting the correct pressure differences to sustain the desired flow conditions in a tower that could be 100 feet high containing 50 or more trays, mechanical malfunctions could occur in the course of the operation. The holes in trays can become blocked through fouling. Trays can be physically displaced due to pressure surges and fall on top of other trays, and where the holes in the trays contain moving parts such as bubble caps, these parts can get stuck.
Malfunctions in the performance of distillation or fractionation are very difficult to identify directly from flow variables such as pressure or temperature. The loss of efficiency in the separation process can be expensive over the long run. Moreover, it is difficult to detect the area of repair from the variables that control the operation of the unit.
It would greatly assist in the operation of a distillation unit or any other processing unit that depended on its operation on an optimized "bubbly medium" if there was a technology that could monitor the operating state of the unit at each tray and alert operators to changes in that state from the ideal spatial separation of phases described above. It would be particularly advantageous if the technique was "non-intrusive" and thus did not require any penetration of the walls of vessel, and would not interfere with any of the operations of the unit. Finally, if the monitoring were continuous, the time between the suspicion of a malfunction and its identification could be reduced to a minimum, with considerable economic advantages. Corrective actions could then be taken, and their success (or failure) monitored. In some cases the unit might have to be removed from service to remedy the problem for example, "fouled trays` in the case of a distillation unit or a plugged sparger in a bubble column, but the repair could be scheduled with minimum lost time. However, it is important to note that portable monitors would also be useful where continuous monitoring is not justified.
Currently, information about the location and status of the internals of a distillation tower and liquid levels while the tower is operating can be supplied by gamma ray techniques. With these techniques a source of gamma rays is moved vertically down or up the tower, and a detector on the opposite side produces a signal that is proportional to the "density" of the material in the path of the beam. It is thus relatively easy to determine if the trays are in the right locations and liquid levels can also be determined. This technique is in common use in refineries and chem plants, but has serious drawbacks. First it is time consuming, not only in the time spent in making the survey, but in the time spent in scheduling the "scan", since it is supplied by outside specialized and licensed personnel. It is expensive; costing thousands of dollars. It is thus common practice to use this technique only when there are strong suspicions that a distillation or fractionation tower is not functioning as expected.
A more subtle weakness of the "gamma scan" technique is that it is only brought into play when there is evidence for a change in tower performance. Thus tower malfunctions that occur during start-up conditions can be overlooked as part of "normal operations".
Finally, there are processing operations which involve chemicals or pressures that require thick walls on the vessel. Under such conditions, or where the vessel containing the process is of a very large diameter, gamma scans lack the sensitivity to detect flow malfunctions within the unit.
There is thus a need for a less intrusive process than the "gamma scan" to monitor the flow state of a fractionation or distillation tower than can be applied and interpreted by refinery personnel. It is unlikely that it will be necessary to supply such the information in real time by a continuous link to a refinery control room. What is important, is the ability to know on demand, how a given unit differs from its past performance and from that of other similar units in other refineries. However, the invention described here can easily function in either mode. Gamma scans, as noted, can only supply information in a "batch" mode.
In the present invention, the magnitude of the signal from an accelerometer attached to the external wall of an atmospheric pipestill or a bubble column or any turbulently flowing liquid containing gas bubbles is used as a sensitive indicator of the flow state of the unit.